Applications of an IEEE 802.11 wireless local area network (especially inside homes) have been increasing and become more and more complicated. Various applications, e.g., smart home automation, entertainment products, etc., have considerably changed the amount of traffic over the home wireless network. The traffic in home is no longer caused by PCs or laptops only, but also by a number of sensors, e.g., temperature sensors, energy meters, multimedia sensors, etc. Such varieties result in a wide range of data rates from several kbps to Mbps in the home traffic, which greatly challenges the IEEE 802.11 wireless local area network.
As well known, the IEEE 802.11 wireless local area network was originally designed for best-effort services. A contention based access is the basis of IEEE 802.11's various traffic control schemes. However this simple access scheme usually causes a high error rate, contention and retransmission frequency as well as unpredictable delays and jitters, all of which greatly lower the quality of a (real-time) voice/video service in the IEEE 802.11 based wireless local area network.
Technical problems with traffic control for the IEEE 802.11 contention based access are listed as follows:
Problem 1: Access Category Limitation of IEEE 802.11.
IEEE 802.11e defines only 4 Access Categories (ACs), that is, AC_BK (Background), AC_BE (Best-Effort), AC_VI (Video) and AC_VO (Voice). For achieving preferential treatment, the existing IEEE 802.11 requires the traffic of a station to be categorized as either AC_VI or AC_VO even if the traffic is quite different from video or voice. Such an approach may cause a higher probability of contention and retransmission in the AC_VI and AC_VO categories. For example, heavy interference may be observed among multiple stations of real-time home automation and security services as well as WiFi-enabled TV, because they all belong to the same access category of AC_VI and all compete for a radio resource at the same priority.
Problem 2: Heavy Signaling Overhead for Traffic Control.
During IEEE 802.11 contention, a contention-free interval may be allocated to a protected traffic source by means of a Clear to Send (CTS) frame. In addition, the unprotected traffic sources may sleep for a period of time in accordance with piggyback information in their acknowledgement (ACK) frames so as to clear the access channel for providing contention-free intervals for the protected traffic source. These existing approaches are not scalable and may lead to a considerable signaling overhead when considering the increasing numbers of traffic sources at home.
Since the contention based access is the basis of IEEE 802.11, the aforementioned problems exist not only in a Distributed Coordination Function (DCF) but also in a Point Coordination Function (PCF) and a Hybrid Coordination Function (HCF).